Electrode Stack For Capacitive Device

ABSTRACT

Capacitive devices are described having electrical interconnects of electrodes which possess efficient electrical contact between current collectors, electrical isolation of electrodes, and/or electrochemical stability, while minimizing the mechanical stress and strain applied to the electrodes. The capacitive devices are adaptable to a wide range of electrode dimensions and electrode stack heights.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalApplication No. 60/004,892, filed on Nov. 30, 2007, which isincorporated by reference herein.

BACKGROUND

1. Field of the Invention

The present invention relates generally to a capacitive device and moreparticularly to a capacitive device comprising an electricalinterconnect useful for electric double layer capacitors and/or forcapacitive deionization.

2. Technical Background

Capacitors, like batteries, store energy in the electrical field betweena pair of oppositely charged conductive plates. Developed more than 250years ago, capacitors are frequently used in electrical circuits asenergy storage devices. In recent years, new families of capacitivedevices have been developed which are based on charge separation of ionsin solution and the formation of electrical double layers.

An electric double layer capacitor (EDLC) is an example of a capacitorthat typically contains porous carbon electrodes (separated via a porousseparator), current collectors and an electrolyte solution. Whenelectric potential is applied to an EDLC cell, ionic current flows dueto the attraction of anions to the positive electrode and cations to thenegative electrode. Electric charge is stored in the electric doublelayer (EDL) formed along the interface between each polarized electrodeand the electrolyte solution.

EDLC designs vary depending on application and can include, for example,standard jelly roll designs, prismatic designs, honeycomb designs,hybrid designs or other designs known in the art. The energy density andthe specific power of an EDLC can be affected by the properties thereof,including the electrode and the electrolyte utilized. With respect tothe electrode, high surface area carbons, carbon nanotubes, activatedcarbon and other forms of carbon and composites have been utilized inmanufacturing such devices. Of these, carbon based electrodes are usedin commercially available devices.

Capacitive Deionization (CDI) is a promising deionization technology,for instance, for the purification of water. In this context, positivelyand negatively charged electrodes are used to attract ions from a streamor bath of fluid. The ions form electric double layers on the surfacesof the electrodes, which are fabricated from some form of high surfacearea material, for example, a form of activated carbon. Afterinteraction with the electrodes during the charging period, the fluidcontains a lower overall ion content and is discharged. A volume ofpurge fluid is then introduced to the electrodes. The electrodes arethen electrically discharged, thus releasing the trapped ions into thepurge fluid. The purge fluid is then diverted into a waste stream andthe process repeated.

Electrically connecting electrodes to a power source is a challengingaspect for EDLC and CDI applications. Typically, electrodes aredelicate, thus mechanical stressing and straining of the electrodesshould be minimized. Minimizing the deformations applied to theelectrodes is difficult, especially while attempting to maximize theelectrical and mechanical integrity of an electrical interconnect to theelectrodes.

U.S. Pat. No. 5,954,937 relates to an interconnection forresorcinol/formaldehyde carbon aerogel/carbon paper sheet electrodes.The fluid flow path is located between the surfaces of the electrodesheets. The active surfaces of these electrode sheets are delicate andshould be protected from mechanical stressing. The electrode sheets arebonded to a current collector, in this case, a titanium sheet using aconductive carbon filled adhesive. The large area of contact between theelectrode sheet and the current collector insure relatively low overallresistance despite the moderately high resistivity of the adhesiveinterface.

U.S. Pat. No. 6,778,378 relates to electrodes which may be rolled fromcarbon and fibrillated polytetrafluoroethylene (PTFE). Electrodes formedin this fashion are thin flexible sheets which can be contacted by highnormal compressive forces. Electrodes may be stacked up with sheets ofcurrent collector material and a separator material and then clampedwith a compressive force to obtain good electrical contact. Bycontrolling which electrodes and current collectors are in physicalcontact, a capacitive cell may be formed.

In commonly owned U.S. Pat. No. 6,214,204, monolithic, low back pressureporous electrodes are made by one of several methods, which includehoneycomb extrusion, casting or molding from a phenolic resin-basedbatch. After curing, these parts are carbonized and activated to createhigh surface area carbon monoliths with good electrical conductivity.

Discs are made and assembled in a stack and spaced such that the discsare electrically isolated from each other. The discs are connected toanode and cathode current collector/bus bar assemblies utilizing wires.

A variety of other approaches to electrically interconnecting electrodeshave been considered in the art with one or more disadvantages asdescribed below. Brazing or soldering alloys typically will notwithstand either the EDLC or the CDI electrochemical environments.Brazing and/or soldering to carbon is difficult due, in part, to the lowstrength of activated carbon. Conductive adhesives formulated usinghighly conductive metal powders are costly and/or are prone tocorrosion. Conductive adhesives formulated using carbon powdersgenerally have insufficient electrical conductivity for use in acapacitor.

Conductive wire or strip leads mechanically fastened around theperimeter of a capacitive device provide adequate performance for smallelectrodes. However the resistive losses introduced by conducting chargearound the circumference of the electrode in a small diameter wire orthin strip lead degrade performance, and no simple means has been foundto use this attachment scheme while incorporating a high efficiencycurrent collector. Also, the logistics of attaching leads to individualelectrodes are not appealing.

It would be advantageous to have a capacitive device comprising anelectrical interconnect to a linear stack of electrodes, which does notjeopardize the mechanical integrity of the electrodes. Also, it would beadvantageous to have the electrical interconnect be electrochemicallyinert. Further, it would be advantageous to develop a capacitive device,comprising interconnected monolithic high surface area carbonelectrodes, which is capable of non-impeded fluid flow through theelectrodes, which is useful for, for example, CDI.

SUMMARY

Capacitive devices, as described herein, address one or more of theabove-mentioned disadvantages of conventional capacitive devices andprovide one or more of the following advantages: efficient electricalcontact between alternating electrodes arranged in series in a linearstack, electrical isolation of adjacent electrodes and electrochemicalstability, while minimizing the mechanical stress and strain applied tothe electrodes. The capacitive devices of the present invention areadaptable to a wide range of electrode dimensions and electrode stackheights.

One embodiment of the invention is a capacitive device comprising planarelectrodes arranged in series. A first electrically conductive carbonmaterial provides electrical contact between at least two alternatingelectrodes, and an electrically insulating material is disposed betweenadjacent electrodes in the series.

Another embodiment of the invention is a method of making a capacitivedevice. The method comprises providing planar electrodes arranged inseries, applying a first electrically conductive carbon material to oneor more of the electrodes such that the first electrically conductivecarbon material provides electrical contact between at least twoalternating electrodes, and applying an electrically insulating materialbetween adjacent electrodes in the series.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from the description or recognizedby practicing the invention as described in the written description andclaims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework tounderstanding the nature and character of the invention as it isclaimed.

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate one or moreembodiment(s) of the invention and together with the description serveto explain the principles and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be understood from the following detailed descriptioneither alone or together with the accompanying drawing figures.

FIG. 1 is an exploded view schematic of features of a capacitive deviceaccording to one embodiment of the invention.

FIG. 2 is a schematic of features of a capacitive device according toone embodiment of the invention.

FIGS. 3 a-3 f are illustrations of exemplary placements of theelectrically conductive carbon material according to some embodiments ofthe invention.

FIGS. 4 a-4 c are illustrations of exemplary placements of theelectrically insulating material according to some embodiments of theinvention.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

FIG. 1 and FIG. 2 show features 100 and 200 respectively of a capacitivedevice, according to one embodiment, comprising planar electrodes 10arranged in series. A first electrically conductive carbon material 12provides electrical contact between at least two alternating electrodes14 and 16, and an electrically insulating material 18 (shown in FIG. 1)is disposed between adjacent electrodes in the series. The electricallyinsulating material 19 shown in FIG. 2 could be used to separate theelectrode on which it is disposed from another electrode in the seriesor to separate the electrode on which it is disposed from an externalhousing.

A capacitive device can be formed by incorporating two or more currentcollectors applying a potential difference to alternating electrodes inthe series, thus forming a capacitive device having a series ofalternating anodes and cathodes. Any means known in the art can be usedto supply the electrical charges to the current collectors.

In one embodiment, the capacitive device, as shown by features 200 inFIG. 2, further comprises a second electrically conductive carbonmaterial 20 which provides electrical contact between at least twodifferent alternating electrodes 22 and 24 in the series. The secondelectrically conductive carbon material is electrically isolated fromthe first electrically conductive carbon material 12.

According to one embodiment, as shown by features 300 of a capacitivedevice, in FIG. 3 a, at least one of the electrically conductive carbonmaterials 12 extends through one or more of the electrodes 10 from afirst planar surface 26 to a second planar surface 28.

Features 301 of a capacitive device are shown in FIG. 3 b. In thisembodiment, the electrically conductive carbon material 12 extendsthrough the second planar surface 28 and protrudes therefrom. Theelectrically conductive carbon material, in one embodiment, also shownin FIG. 3 b, the electrically conductive carbon material is equallydistributed in height extending from a first planar surface 26 and asecond planar surface 28 of at least one electrode.

In another embodiment, as shown by features 302 and 303 in FIG. 3 c andFIG. 3 d, the electrically conductive carbon material 12 protrudes fromthe first planar surface 26 and the second planar surface 28. Theelectrically conductive carbon material, according to some embodiments,can protrude from either the first planar surface or the second planarsurface or both planar surfaces. The protrusions do not have to beequally distributed.

In another embodiment, the electrically conductive carbon material canbe the same material as one or more of the electrode material(s). Theelectrode 10 and the electrically conductive carbon material 12, in oneembodiment, can be a single molded piece as shown by features 304 of acapacitive device shown in FIG. 3 e. Multiple protruding regions ofelectrically conductive carbon materials can be formed in a singlecomposite element, in some embodiments.

According to another embodiment, the electrically conductive carbonmaterial 12 can be oriented along one or more edges of one or more ofthe electrodes 10, as shown by features 305 of a capacitive device shownin FIG. 3 f.

In one embodiment, the electrically conductive carbon material isselected from a carbon paper, carbon fibers, carbon particles, carbonfoam and combinations thereof.

Electrodes for the stack can be manufactured by methods known to thoseskilled in the art. Requirements for the electrode include goodelectrical conductivity and double layer capacitance, resistance tocorrosion or electrochemical corrosion in electrolyte, and rigidity forpackaging robustness. For example, the electrodes can have a layeredstructure whereby a backplane is incorporated in the electrode with EDLCmaterials adhered to the backplane surface.

Alternately, the electrodes could be a composite material where the EDLCis combined with a polymeric binder. Alternately, the EDLC can bedirectly incorporated into the structure of a porous backplane.

The electrodes can be any shape or size providing the electrodecomprises a first face, an opposing second face and a thickness definedby an outer surface extending from the first face to the opposing secondface. An electrode having flattened areas on the outer surface mayfacilitate improved electrical connections to a current collector alsohaving a surface with flattened areas contacting the electrodes. Theelectrodes can be, for example, polygonal, circular, cylindrical,square, triangular, pentagonal, hexagonal or a combination thereof.

The electrodes can readily incorporate a hole or an array of holes, forexample, punched holes or other designs, which can be used to enable ahybrid parallel/transverse flow through CDI cell design. In oneembodiment, one or more of the electrodes comprise one or more holesextending through the thickness of the electrode(s). The hole(s) can be,for example, from 1 mm to 10 mm in diameter.

The capacitive device, according to one embodiment, comprises anelectrically insulating material disposed between adjacent electrodes.One function of the electrically insulating material is to electricallyisolate opposing pairs of anodes and cathodes. The electricallyinsulating material should be electrochemically inert and electricallyinsulating. In addition, this material should also be mechanically rigidsuch that it can withstand mechanical compression.

In one embodiment, as shown by features 401 in FIG. 4 b, theelectrically insulating material 18 protrudes from a first planarsurface 26 of at least one electrode 10 in the series.

In another embodiment, as shown by features 402 in FIG. 4 c, theelectrically insulating material 18 protrudes from a first planarsurface 26 of at least one electrode 10 in the series and from a secondplanar surface 28 of at least one electrode 10 in the series.

In another embodiment, as shown by features 400 in FIG. 4 a theelectrically insulating material 18 extends through one or more of theplanar electrodes 10.

The electrically insulating material should be selected such that itdoes not outgas any components which may poison the capacitance of asurrounding electric double layer. As an example, some silicones containorganic impurities which, upon leaching, can adsorb onto andconsequently poison carbon surfaces. Other polymers, including selectedsilicones, could be used such that poisoning is not a concern. Exemplaryinsulating materials may be selected from, for example, a silicone, apolymer, an elastomer, natural rubber, silicone rubber, butyl rubber,polybutadiene, nitrile rubber, polyurethane rubber, fluoroelastomers andcombinations thereof.

The electrically insulating material may be in any form appropriate foruse as an insulating material between adjacent electrodes. For example,it may be in the form of a sheet, a bead or beads, one or more discreteregions, and combinations thereof. According to one embodiment, theelectrically insulating material is in a form selected from a sheet, aporous sheet, for example, a woven fabric or non-woven sheet, one ormore discrete regions, for example, a strip or strips, a bead or beads,and combinations thereof. In some embodiments, the electricallyinsulating material is planar, for example, one or more discrete planarsheets.

The electrically insulating material can be formed using any methodknown in the art, for example, a method selected from lithography,photolithography, molding, printing, and combinations thereof.

When the electrically insulating material is in the form of discreteregions, such as polymer beads, the process used to pattern the polymerbeads could be based on printing, molding, or photolithography, forinstance. The number, spacing and dimensions of the beads will depend oncharacteristics of both the electrodes and the material composition. Thenumber and spacing of beads will be determined by the stiffness of theelectrodes, the amount of compressive force applied to the stack, and adetermination of how much double layer capacity loss can be toleratedsince the bead material detracts from the available capacitive geometricsurface area. The dimensions of the beads will be primarily a functionof the mechanical properties of the polymer and the compressive forcesto be applied.

Another embodiment of the invention is a method of making a capacitivedevice. The method comprises providing planar electrodes arranged inseries, applying a first electrically conductive carbon material to oneor more of the electrodes such that the first electrically conductivecarbon material provides electrical contact between at least twoalternating electrodes, and applying an electrically insulating materialbetween adjacent electrodes in the series.

In the embodiment shown in FIG. 1 and FIG. 2, strips of electricallyconductive carbon material 12, for example, carbon paper are bonded tothe edge of a larger sheet of carbon paper and carbonized to form anintegrated current collector. Activated carbon is bonded to bothsurfaces of the current collector. Electrically insulating materials,for example, polymer insulator/separator features are molded/bonded tothe surface of the electrode. Electrodes 10 are stacked in alternating“left-right” orientation to form an anode-cathode array. Compression isapplied to complete electrical interconnection of the current collectorassemblies.

The electrically conductive carbon material(s) used to build up a thickedge on the current collector of one or more of the electrodes should beconductive, electrochemically inert, and possess reasonable mechanicaldurability. A carbon-based material, specifically one that is graphiticin nature, is advantageous for its electrochemical inertness and highelectrical conductivity. The mechanical durability of this materialensures that the spacing of the electrode capacitive gap is well-definedspatially. Furthermore, it is advantageous that the carbon material beporous such that it can be filled with a thermoplastic or a carbonizablematerial for the purpose of bonding it to the current collector. If acarbonizable resin or other polymer is used for bonding, then aconductive, porous, graphite-based paper is one example of a materialthat would meet the requirements of the material needed to form the busbar on the edge of the electrodes.

In one embodiment, the application of the first electrically conductivecarbon material comprises bonding one or more sheets of the electricallyconductive carbon material to each electrode.

In one embodiment, the method further comprises subsequently bonding oneor more sheets of the electrically conductive carbon material to the oneor more sheets bonded to each electrode to form a stack of bondeddiscrete sheets.

In one embodiment, the applying an electrically insulating materialbetween adjacent electrodes in the series comprises forming theelectrically insulating material by lithography.

A lithography process can provide automated location of the electricallyinsulating material on one or more of the electrodes. For example, aphotoresist that is sensitive to light, typically in the UV range of thespectrum can be used to pattern the electrically insulating material ina certain shape and/or pattern and/or location using a mask having thedesired design. The mask can be reused and can be designed andmanufactured, for example, using high quality glass or quartz. Smallfeatures, for example, in the micron range can be produced on one ormore of the electrodes.

For large features a mask aligner and a simple UV lamp with collimatedlight, for example, can be used to pattern to a certain precision in themicron range. This can make the manufacturing process more accessibleand of low cost.

The photoresist can be deposited by, for example, spin coating, tapecasting or other methods known in the art. The thickness of the resistcan be controlled, for example, by the viscosity of the photoresistand/or by the method of deposition. Certain photoresists, such as theSU-8 photoresist family, commercially available from MicroChem Corp.,Newton, Mass., can provide thicknesses, for example, 50 μm or more withhigh uniformity.

Prior to irradiation with UV light, the deposited photoresist should bebaked for a certain length of time at moderate temperatures asrecommended by the photoresist manufacturer.

After irradiation with UV light the area of interest is formed,depending on the mask and photoresist choice, by developing thephotoresist with a developer that is matched specifically to thephotoresist chosen. The photoresist can be of positive type or negativetype depending if one wants to design a mask that maintains or removesthe area where the light reaches the substrate, in this case one or moreof the electrodes. Other polymers such as polyimide can also be madephotosensitive and can be used as the electrically insulating material,several of which are commercially available.

Moreover, a photoresist of the lift-off resist (LOR) can be used topattern other materials. In this embodiment, the photoresist ispatterned with openings on the top of its surface for the deposition ofthe electrically insulating material. The electrically insulatingmaterial, in this embodiment, can be another thick polymer or othermaterial having similar properties as photoresist. In the developingprocess the LOR is removed from the substrate and lifts the undesiredpart of the material deposited. As a result only the desiredelectrically insulating material deposited remains.

The method, according to one embodiment, further comprises applying asecond electrically conductive carbon material such that the secondelectrically conductive carbon material provides electrical contactbetween at least two different alternating electrodes in the series andis electrically isolated from the first electrically conductive carbonmaterial.

EXAMPLE I

Strips of electrically conductive porous carbon sheet (preferably paper)are bonded along one or more edges of the electrode's integratedconductive porous carbon sheet current collector using a thermoplasticor a carbonizable polymer such as a curable resin. The polymer binder isthen cured (if necessary) and carbonized, forming a thick conductiveelectrical contact strip. The thickness of this strip should be greatenough to define the separation of the electrodes in the final array.The layers of high surface area carbon powder which form the EDLCelectrode layer may be bonded to both sides of the sheet before, duringor after this process (obviously after this process if the adhesive isnot going to be carbonized). Small electrically insulating polymercontact beads may then be deposited to one or both sides of the EDLCelectrode layer to serve as separators, providing electrical isolationand enforcing the thickness of the fluidic chamber between electrodes. Anumber of these electrodes are then stacked up, with the contact stripsalternately oriented along opposite edges of the stack to develop aninterdigitated array of alternately charged electrodes. Mechanicalpressure is then applied to the stack normal to the surface of thebonded strips, either through use of a clamp or the use of a rigidhousing. This pressure effectively forms two monolithic currentcollectors by enforcing good electrical contact throughout the twoelectrically isolated electrode arrays. Electrical connection to theoutside world may then be made by either mechanical pressure orconductive adhesive bonding to the compressed current collector stacks.

Capacitive devices, for example, CDI cells produced according to theinvention possess one or more of the following desirablecharacteristics: the use of integrated contact strips, for example,along the electrode edges provides a simple means for electricalconnection as well as a means of mechanically defining the capacitivegaps between electrodes, integration of the electrically insulatingmaterial, for example, spacers across the electrode faces ensureselectrical isolation between anode and cathode elements whilesimultaneously defining the capacitive gaps between electrodes, ease ofassembly—the manufactured anode and cathode elements with electricalcontacts and spacers are identical except that they are rotated 180°during the assembly process, and ease of packaging—once the stack isassembled, simple mechanical pressure is applied to ensure goodelectrical contact between anodes and cathodes, respectively.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A capacitive device comprising: planar electrodes arranged in series;a first electrically conductive carbon material providing electricalcontact between at least two alternating electrodes; and an electricallyinsulating material disposed between adjacent electrodes in the series.2. The capacitive device according to claim 1, further comprising asecond electrically conductive carbon material providing electricalcontact between at least two different alternating electrodes in theseries and electrically isolated from the first electrically conductivecarbon material.
 3. The capacitive device according to claim 1, whereinat least one of the electrically conductive carbon materials extendsthrough one or more of the electrodes from a first planar surface to asecond planar surface.
 4. The capacitive device according to claim 3,wherein the electrically conductive carbon material extends through thesecond planar surface and protrudes therefrom.
 5. The capacitive deviceaccording to claim 1, wherein the electrically conductive carbonmaterial is equally distributed in height extending from a first planarsurface and a second planar surface of at least one electrode.
 6. Thecapacitive device according to claim 1, wherein the electricallyinsulating material is in a form selected from a sheet, a porous sheet,one or more discrete regions and combinations thereof.
 7. The capacitivedevice according to claim 1, wherein the electrically insulatingmaterial is selected from a silicone, a polymer, an elastomer, naturalrubber, silicone rubber, butyl rubber, polybutadiene, nitrile rubber,polyurethane rubber, fluoroelastomers and combinations thereof.
 8. Thecapacitive device according to claim 1, wherein the electricallyinsulating material protrudes from a first planar surface of at leastone electrode in the series.
 9. The capacitive device according to claim8, wherein the electrically insulating material extends through one ormore of the planar electrodes.
 10. The capacitive device according toclaim 1, wherein the electrically insulating material is bonded to oneor more of the planar electrodes.
 11. The capacitive device according toclaim 1, wherein one or more of the electrodes comprises a holeextending through the thickness of the electrode.
 12. The capacitivedevice according to claim 11, wherein the hole is from 1 mm to 10 mm indiameter.
 13. A method of making a capacitive device, the methodcomprising: providing planar electrodes arranged in series; applying afirst electrically conductive carbon material to one or more of theelectrodes such that the first electrically conductive carbon materialprovides electrical contact between at least two alternating electrodes;and applying an electrically insulating material between adjacentelectrodes in the series.
 14. The method according to claim 13, whereinthe applying the first electrically conductive carbon material comprisesbonding one or more sheets of the electrically conductive carbonmaterial to each electrode.
 15. The method according to claim 14,further comprising subsequently bonding one or more sheets of theelectrically conductive carbon material to the one or more sheets bondedto each electrode to form a stack of bonded discrete sheets.
 16. Themethod according to claim 15, wherein the applying an electricallyinsulating material between adjacent electrodes in the series comprisesforming the electrically insulating material by a method selected fromlithography, photolithography, molding, printing, and combinationsthereof.
 17. The method according to claim 13, further comprisingapplying a second electrically conductive carbon material such that thesecond electrically conductive carbon material provides electricalcontact between at least two different alternating electrodes in theseries and is electrically isolated from the first electricallyconductive carbon material.